I’ve been guilty of perpetuating a myth. Not long ago I wrote an article in which I said installing insulation, “cavities [should be] filled completely with as little compression as possible.” But is compression really such a bad thing? Here on GBA, commenter Dana Dorsett wrote, “Compression of batts is fine (resulting in a higher R/inch due to the higher density) as long as the cavity is completely filled.”
He’s right. Compression isn’t the problem. Incompletely filled cavities are a problem. Gaps are a problem. But you can compress fiberglass insulation quite a bit and it still works just fine. The North American Insulation Manufacturers Association (NAIMA) has a little two-page document about compressing fiberglass insulation. Here’s what NAIMA says: “When you compress fiber glass batt insulation, the R-value per inch goes up, but the overall R-value goes down because you have less inches or thickness of insulation.”
The document includes a general chart for how to tell what your R-value is with different levels of compression. Owens Corning also has a compression chart for R-value (see Image #2, below).
So, you don’t get the full R-value on the label, but the insulation still works perfectly well if all you’ve done is compress it. Of course there are limits. If you use a hydraulic press to compress it so much that it approaches the density of solid glass, things change. We’re talking about reasonable amounts of compression.
Here’s something you may not know. The standard R-19 fiberglass batt is 6.25 inches thick. If you put that batt in a closed 2×6 wall, it will be compressed 0.75 inch because a 2×6 is 5.5 inches deep. That means the batt labeled R-19 really gives you R-18 in a closed cavity.
One place where you’re pretty…